1. Field of the Invention
This invention relates in general to well production systems, and in particular to a hydraulic seal between a tubing hanger and a wellhead member for providing hydraulic fluid to a hydraulically operated downhole safety valve.
2. Description of the Prior Art
Downhole safety valves are often used in well production systems. These downhole safety valves are connected into the production tubing string and are designed to shut off flow through the production tubing string in case of a malfunction so as to avoid a blowout.
Most downhole safety valves are hydraulically operated. Hydraulic pressure maintains the valve in the open position. Removal or interruption of the hydraulic pressure causes the safety valve to shut off flow through the production tubing string. In case of an emergency, the hydraulic pressure can be either intentionally interrupted, or might be interrupted by the catastrophe itself, thus avoiding a blowout.
A hydraulic line usually extends to the downhole safety valve from the surface to provide the safety valve with hydraulic fluid pressure. The hydraulic fluid line usually extends adjacent the production tubing string. The connection at the surface between the hydraulic fluid line and the hydraulic fluid source is often a concern because the hydraulic line usually reaches the surface on the inside of the wellhead members and must make a connection with the exterior of the wellhead members.
One common technique for providing hydraulic fluid from outside the wellhead to the hydraulic fluid line extending to the downhole safety valve is to connect the top of the hydraulic fluid line to a tubing hanger hydraulic fluid passage in the tubing hanger. The tubing hanger hydraulic fluid passage has a lateral portion which communicates with a wellhead member hydraulic fluid passage extending laterally through the wellhead member within which the tubing hanger is landed. The wellhead member hydraulic fluid passage is in turn connected to the hydraulic fluid source.
The sealing connection between the tubing hanger hydraulic fluid passage and the wellhead member hydraulic fluid passage is accomplished by the use of two circumferential annular seals. One seal extends around the circumference of the tubing hanger just above the lateral openings of the hydraulic fluid passages in the tubing hanger and wellhead member. Another seal extends around the circumference of the tubing hanger just below the lateral openings of the hydraulic fluid passages of the tubing hanger and wellhead member. Each of these two seals forms an annular seal between the circumference of the tubing hanger and the wellhead member so as to isolate an annular void between the tubing hanger and the wellhead member through which the hydraulic fluid passages can communicate.
Where the application allows, another common technique for providing hydraulic fluid from outside the wellhead to the hydraulic fluid line extending to the downhole safety valve is to obviate the need for the annular seals by providing a screwed connection directly to the tubing hanger hydraulic fluid passage. The hydraulic fluid source line is screwed through the wellhead and into a radial threaded opening of the tubing hanger hydraulic fluid passage.
Another common technique for providing hydraulic fluid from outside the wellhead to the hydraulic fluid line extending to the downhole safety valve is to have a vertical tubing hanger hydraulic fluid passage which extends the full length of the tubing hanger. The hydraulic line from the downhole safety valve connects into the bottom opening of the tubing hanger hydraulic passage. A stab from the top of the wellhead connects to the upper opening of the tubing hanger hydraulic fluid passage, thus providing hydraulic fluid to the downhole safety valve.
The annular circumferential seal method described above is the preferred method from an installation perspective because once the tubing hanger is landed into the wellhead, the hydraulic connection is complete. However, due to the large diameters involved, and the harsh conditions encountered during installation and operation, it is difficult to create reliable circumferential annular seals between the tubing hanger and wellhead member, especially for high pressure applications. Also, since the tubing hanger is often landed into the wellhead member under imperfect conditions, preventing seal damage during, and prior to, installation is also a concern.
The stab methods, both radial and vertical, described above are more suited for high pressure applications, however, installation is more complex and the stab line extending through the wellhead creates additional concerns.
A need exists for a reliable, high pressure, easy to install connection for providing hydraulic fluid from outside the wellhead to the hydraulically operated downhole safety valve.